Hot-dip Al—Zn—Mg—Si coated steel sheet and method of producing same
Provided is a hot-dip Al—Zn—Mg—Si coated steel sheet having good corrosion resistance in flat parts and edge parts, and also having excellent worked part corrosion resistance. The hot-dip Al—Zn—Mg—Si coated steel sheet includes a base steel sheet and a hot-dip coating on a surface of the base steel sheet. The hot-dip coating includes an interfacial alloy layer present at an interface with the base steel sheet and a main layer present on the interfacial alloy layer, and contains from 25 mass % to 80 mass % of Al, from greater than 0.6 mass % to 15 mass % of Si, and from greater than 0.1 mass % to 25 mass % of Mg. The Mg content and Si content in the hot-dip coating satisfy formula (1): M Mg /( M Si −0.6)>1.7 (1) where M Mg represents the Mg content (mass %) and M Si represents the Si content (mass %).
1. A hot-dip Al—Zn—Mg—Si coated steel sheet comprising
a base steel sheet and a hot-dip coating on a surface of the base steel sheet, wherein
the hot-dip coating includes an interfacial alloy layer present at an interface with the base steel sheet and a main layer present on the interfacial alloy layer, and consists of from 25 mass % to 80 mass % of Al, from greater than 0.6 mass % to 15 mass % of Si, from greater than 0.1 mass % to 25 mass % of Mg, optionally from 0.2 mass % to 25 mass % of Ca, optionally one or more selected from Mn, V, Mo, Ti, Sr, Ni, Co, Sb, and B in a total amount of from 0.01 mass % to 10 mass %, and the balance being Zn and incidental impurities, and
Mg content and Si content in the hot-dip coating satisfy formula (1):
M Mg /( M Si −0.6)>1.7 (1)
where M Mg represents the Mg content in mass % and M Si represents the Si content in mass %, and wherein
the main layer includes an α-Al phase dendritic region, and a mean dendrite diameter of the α-Al phase dendritic region and a thickness of the hot-dip coating satisfy formula (2):
t/d≥ 1.5 (2)
where t represents the thickness of the hot-dip coating in μm and d represents the mean dendrite diameter in μm.
2. The hot-dip Al—Zn—Mg—Si coated steel sheet according to claim 1 , wherein
the main layer contains Mg 2 Si, and Mg 2 Si content in the main layer is 1.0 mass % or more.
3. The hot-dip Al—Zn—Mg—Si coated steel sheet according to claim 2 , wherein
the content of Al in the hot-dip coating is from 25 mass % to 80 mass %,
the content of Si in the hot-dip coating is from greater than 2.3 mass % to 5 mass %, and
the content of Mg in the hot-dip coating is from 3 mass % to 10 mass %.
4. The hot-dip Al—Zn—Mg—Si coated steel sheet according to claim 2 , wherein
the content of Al in the hot-dip coating is from 25 mass % to 80 mass %,
the content of Si in the hot-dip coating is from greater than 0.6 mass % to 15 mass %, and
the content of Mg in the hot-dip coating is from greater than 5 mass % to 10 mass %.
5. The hot-dip Al—Zn—Mg—Si coated steel sheet according to claim 1 , wherein
the main layer contains Mg 2 Si, and an area ratio of Mg 2 Si in a cross-section of the main layer is 1% or more.
6. The hot-dip Al—Zn—Mg—Si coated steel sheet according to claim 5 , wherein
the content of Al in the hot-dip coating is from 25 mass % to 80 mass %,
the content of Si in the hot-dip coating is from greater than 2.3 mass % to 5 mass %, and
the content of Mg in the hot-dip coating is from 3 mass % to 10 mass %.
7. The hot-dip Al—Zn—Mg—Si coated steel sheet according to claim 5 , wherein
the content of Al in the hot-dip coating is from 25 mass % to 80 mass %,
the content of Si in the hot-dip coating is from greater than 0.6 mass % to 15 mass %, and
the content of Mg in the hot-dip coating is from greater than 5 mass % to 10 mass %.
8. The hot-dip Al—Zn—Mg—Si coated steel sheet according to claim 1 , wherein
the main layer contains Mg 2 Si, and according to X-ray diffraction analysis, an intensity ratio of Mg 2 Si (111) planes having an interplanar spacing d of 0.367 nm relative to Al (200) planes having an interplanar spacing d of 0.202 nm is 0.01 or more.
9. The hot-dip Al—Zn—Mg—Si coated steel sheet according to claim 8 , wherein
the content of Al in the hot-dip coating is from 25 mass % to 80 mass %,
the content of Si in the hot-dip coating is from greater than 2.3 mass % to 5 mass %, and
the content of Mg in the hot-dip coating is from 3 mass % to 10 mass %.
10. The hot-dip Al—Zn—Mg—Si coated steel sheet according to claim 8 , wherein
the content of Al in the hot-dip coating is from 25 mass % to 80 mass %,
the content of Si in the hot-dip coating is from greater than 0.6 mass % to 15 mass %, and
the content of Mg in the hot-dip coating is from greater than 5 mass % to 10 mass %.
11. The hot-dip Al—Zn—Mg—Si coated steel sheet according to claim 1 , wherein
the interfacial alloy layer has a thickness of 1 μm or less.
12. The hot-dip Al—Zn—Mg—Si coated steel sheet according to claim 11 , wherein
the content of Al in the hot-dip coating is from 25 mass % to 80 mass %,
the content of Si in the hot-dip coating is from greater than 2.3 mass % to 5 mass %, and
the content of Mg in the hot-dip coating is from 3 mass % to 10 mass %.
13. The hot-dip Al—Zn—Mg—Si coated steel sheet according to claim 11 , wherein
the content of Al in the hot-dip coating is from 25 mass % to 80 mass %,
the content of Si in the hot-dip coating is from greater than 0.6 mass % to 15 mass %, and
the content of Mg in the hot-dip coating is from greater than 5 mass % to 10 mass %.
14. The hot-dip Al—Zn—Mg—Si coated steel sheet according to claim 1 , wherein
the content of Al in the hot-dip coating is from 25 mass % to 80 mass %,
the content of Si in the hot-dip coating is from greater than 2.3 mass % to 5 mass %, and
the content of Mg in the hot-dip coating is from 3 mass % to 10 mass %.
15. The hot-dip Al—Zn—Mg—Si coated steel sheet according to claim 1 , wherein
the content of Al in the hot-dip coating is from 25 mass % to 80 mass %,
the content of Si in the hot-dip coating is from greater than 0.6 mass % to 15 mass %, and
the content of Mg in the hot-dip coating is from greater than 5 mass % to 10 mass %.
16. The hot-dip Al—Zn—Mg—Si coated steel sheet according to claim 1 , wherein
the hot-dip coating has a thickness of 27 μm or less.
17. A method of producing the hot-dip Al—Zn—Mg—Si coated steel sheet of claim 1 , comprising
hot-dip coating a base steel sheet by immersing the base steel sheet in a molten bath consisting of from 25 mass % to 80 mass % of Al, from greater than 0.6 mass % to 15 mass % of Si, and from greater than 0.1 mass % to 25 mass % of Mg, optionally from 0.2 mass % to 25 mass % of Ca, optionally one or more selected from Mn, V, Mo, Ti, Sr, Ni, Co, Sb, and B in a total amount of from 0.01 mass % to 10 mass %, the balance being Zn and incidental impurities,
subsequently cooling a resultant hot-dip coated steel sheet to a first cooling temperature at an average cooling rate of less than 10° C./sec, the first cooling temperature being no higher than a bath temperature of the molten bath and no lower than 50° C. below the bath temperature, and
then cooling the hot-dip coated steel sheet from the first cooling temperature to 380° C. at an average cooling rate of 10° C./sec or more.